1. Field of the Invention
The present invention relates to an information recording and reproducing system for recording and reproducing information on a magneto-optical disk.
2. Description of the Prior Art
Along with an increase in the capacity and operation speed of electronic computers in recent years, high density and large capacity for the memory device that constitutes the principal part of computers are being demanded to an increasingly high degree. With respect to such memory devices, there are known, for instance, an optical disk file device which records and reproduces information as a bit sequence that can be read optically, and a magnetic recording device which carries out recording of information on a recording medium with magnetic layers, by magnetically inverting magnetization, depending upon the information. Also known are erasable magneto-optical disk devices which record and erase information by means of irradiation of laser light and an application of a magnetic field, and reproduces information by irradiation of laser light. Among the various memory devices, one which is attracting the attention most and studied most actively is the information recording and reproducing device that makes use of an optical disk. This device records information by irradiating an optical disk with a laser beam that is squeezed to a spot with a diameter of about 1 .mu.m to form a bit that has a width of 0.6 to 1 .mu.m and a length of 1 to 2 .mu.m. On the other hand, reproduction of recorded information is carried out by detecting the changes in the reflected or transmitted light from the bits recorded on the optical disk.
As to the optical disk recording media to be used for such a device, there exists a recording medium of direct read after write (DRAW) type for which it is impossible to erase and re-record information, and a recording medium of erasable type for which it is possible to re-record (erase or record) information a plurality of times. The information recording and reproducing device that makes use of a recording medium of the DRAW type has an advantage because of its impossibility of rewriting information. However, from the viewpoint of economy and effectiveness of the medium, recording medium of an erasable type is superior.
A recording medium of the erasable type has an amorphous alloy film that is composed of rare earth elements such as Gd, Td, Dy, and Ho and transition metals such as Fe and Co, as the recording film. From the magnetically ordered condition maintained at room temperature, and with the easy axis of magnetization in the direction perpendicular to the magneto-optical film, a magnetically disordered state can be obtained by irradiating the recording film (perpendicularly magnetized film) with a laser beam or the like.
The magnetically ordered and disordered states here means the conditions in which the coercive force of the easy axis of magnetization are securely held and decreased, respectively. If an external magnetic field with a predetermined direction is applied to the recording film when it is in a magnetically disordered state, the easy axis of magnetization of the recording film rotates toward the direction of the magnetic field applied. In this configuration, it becomes possible to carry out recording or erasing.
Now, the method of recording information on the recording film, which has all of its magnetic domains arranged in the same direction (for example, in the downward direction) in a state where there is no information recorded, will be described in detail.
A laser beam, which is modulated in accordance with the information, is focused to a size with a diameter of about 1 .mu.m, and is made to irradiate the recording film to heat it locally with the energy of the laser beam to reduce the coercive force of that portion of the film. In this case, by applying an external magnetic field (usually, less the 1 kOe) with a direction which is opposite to the initial direction (downward direction) of magnetization, the local direction of magnetization alone will be reversed. A portion of the recording film other than the portion that is irradiated by the laser beam will not undergo a change in the initial direction of magnetization (inversion) even under an application of an external magnetic field. Therefore, there will be formed a pattern that has a different direction (upward direction) of magnetization within a uniform field of magnetization. That is, it corresponds to a recording of information.
Next, a method of erasing previously recorded information by means of a principle which is nearly the same as in the above will be described. Namely, one needs only to select the direction of a magnetic field to be opposite to the direction of the magnetic field that was applied at the time of recording the information. For instance, by irradiating the recorded portion of the recording film with a laser beam and by applying an external magnetic field with a direction which is opposite to that at the time of recording, the direction of magnetization returns to the state which is the same as prior to recording, information previously recorded is erased.
Further, for reproducing information, differing from the case of recording and erasing, use will be made of the rotation of a plane of polarization of the laser beam. On the recording film (perpendicularly magnetized film) of the recording medium, there are disposed magnetic domains perpendicularly. When the film is irradiated by a laser beam, the direction of polarization of the reflected (or transmitted) light rotates according to the direction of the magnetic domain. In other words, if a linearly polarized laser beam irradiates the surface of the film, the plane of polarization of the reflected light rotates slightly depending upon the direction of magnetization of the film. The rotation of the plane of polarization is detected by a light detector and signal, due to the direction of magnetization of the film, is converted to the intensity (1 or 0) of the reflected light, thus reproducing the information.
As stated above, recording, reproducing, and erasing of information are performed by making use of the sense of magnetization of each magnetic domain.
With respect to a memory with the recording film having the above mentioned features, the erasable medium is called a magneto-optical disc. Although this magneto-optical disc is erasable, it is inappropriate to carry out writing operation directly on an area which has already undergone the previous writing operations, since the domain inversed by the previous operations is left as it was. Because of this, an erasing operation on the area of the medium where writing is made is necessary in advance, which would allow for new information to be written on the area. Of course where only one head is available, it takes twice the time to complete a re-recording operation on the recorded medium.
As to methods for rewriting information, there exists, for example, a magneto-optical device for recording, reproducing, and erasing as is disclosed in Japanese Patent No. 59-217250. This device has an optical system for recording which carries out recording of desired information, and another optical system for erasing and reproducing which erases unnecessary information or reproduces information which was written by the recording optical system. With such an arrangement, it becomes possible to partially erase unnecessary information alone, and at the same time, to record new information while erasing old information.
However, in attempting to apply such a concept to a practical device, there still remains certain unsolved technical problems. For example,
1. The provision of two heads requires a large hardware and a sophisticated control system.
2. In carrying out reproducing, it can perform only signal reproduction which is entirely identical to the case of ordinary optical system with one system, such that effective utilization of the system is not at all realized in spite of the fact that two optical systems are provided in the above patent disclosure.
3. Clearly, there has not been a sufficient response for information that is transported successively (continuously flowing information).
4. The waiting time for rotation, in accessing the optical system to the position where rewriting is desired, is too long.
5. In addition, the size of the recording bits for such a device is extremely small, being on the order of one micrometer. Accordingly, even dust, scratches, pinholes, etc., could become the cause of the faulty signals in recording or reproducing, which results in large error rates compared with the case of a magnetic disk and the like. Thus, the magneto-optical disk device is not put to practical use unless these technical problems are resolved.